A plurality of systems uses a battery implemented as a battery pack or battery array; including a plurality of battery cells connected in series with each other.
When such a battery cell is charged to a much higher voltage or a much lower voltage than the voltage within a rated charge range, it may be dangerous.
Imbalance in the charged state of battery cells is caused by various factors, and occurs during the manufacture of batteries or the charge/discharge of batteries. In the case of lithium ion cells, the manufacture of cells is strictly controlled within a company to minimize the differences between the capacities of the cells of a battery array. However, imbalance or inequality between cells may occur due to various factors, regardless of the states of the cells, in which balance or equality is maintained after the cells are initially manufactured.
The factors influencing the imbalance of cells may include, for example, the chemical reactions, impedances and self-discharge rates of respective cells, reduction of the capacities of the cells, variation in the operating temperatures of the cells, and other types of variation between the cells.
Inconsistency in the temperature of cells is an important factor responsible for causing imbalance in cells. For example, “self-discharge” is caused in a battery cell, and is a function of a battery temperature. A battery having a high temperature typically has a self-discharge rate higher than that of a battery having a low temperature. As a result, the battery having a high temperature exhibits a lower charged state than the battery having a low temperature, with the passage of time.
Imbalance is a very series problem in the charged state of a battery. For example, this problem may typically occur in electric vehicles, and the capability of a battery to supply energy is limited by the battery cell having the lowest charged state.
If one of series-connected batteries is fully consumed, other battery cells lose the ability to continue to supply energy. This is the same even if the other battery cells of the battery still have the ability to supply power. Therefore, an imbalance in the charged state of battery cells reduces the power supply capability of the battery.
Of course, the above description does not mean that when one or more battery cells are consumed the supply of power by the remaining battery cells is completely impossible. However, it means that, only in the case of series connection, even if one or more battery cells are fully consumed, the battery can be continuously used as long as charge remains in the remaining battery cells, but, in that case, voltage having a reversed polarity is generated in the battery cell which has been fully discharged, and, as a result, the battery cell may be in danger of explosion due to the overheating thereof, or due to the generation of gas, and thus the battery loses power supply capability.
Various methods of correcting an imbalance in the charged state of battery cells have been proposed, and one of the methods is shown in FIG. 1.
FIG. 1 is a diagram showing the construction of a conventional centralized charge equalization apparatus having series-connected battery cells.
Referring to FIG. 1, the conventional centralized charge equalization apparatus having series-connected battery cells is constructed such that the common core of a transformer T is provided, a single primary winding M1 is wound around the common core, and a number of secondary windings M21 to M2N corresponding to the number of battery cells B1 to BN is wound around the common core. The secondary windings M21 to M2N have the same number of turns and have the same polarity, that is, a negative polarity. In this case, the fact that the polarity of the secondary windings M21 to M2N is a negative polarity means that dots are marked on the lower portions of respective windings.
Only a single primary winding M1 is wound around the common core of the transformer T, and a switch S is connected in series with the primary winding M1, which has polarity opposite that of the secondary windings M21 to M2N.
The switch S functions to turn on/off current flowing through the primary winding M1, connected in series with the switch S, and performs ON/OFF operation in response to a control signal output from a voltage sensing and switch drive signal generation unit 100.
Rectifier diodes D1 to DN are connected in series with respective secondary windings M21 to M2N wound around the common core of the transformer T, and battery cells B1 to BN are connected in parallel with respective secondary windings M21 to M2N.
Further, the battery cells B1 to BN are connected to the voltage sensing and switch drive signal generation unit 100.
The voltage sensing and switch drive signal generation unit 100 senses the voltages of respective battery cells B1 to BN, connected thereto, and turns on/off the switch S on the basis of the sensed voltages, thus maintaining the voltages of the series-connected battery cells B1 to BN at a uniform voltage.
That is, the voltage sensing and switch drive signal generation unit 100 senses the voltages of respective battery cells B1 to BN, and turns on the switch S when the voltage of a specific battery cell B1 to BN is higher than a preset voltage. Charge is discharged from the series-connected battery cells B1 to BN and is converted into magnetic energy by the transformer T, and the magnetic energy is stored in the transformer T. When the switch S is turned off, the magnetic energy is converted back into charge, and the charge moves to respective battery cells B1 to BN through the secondary windings M21 to M2N and the rectifier diodes D1 to DN. At this time, a small amount of charge moves to a battery cell B1 to BN having a high voltage, whereas a large amount of charge moves to a battery cell B1 to BN having a low voltage, thus equalizing the charge of the battery cells.
The conventional centralized charge equalization apparatus is advantageous in that it controls the flow of charge using only a single switch. However, the conventional centralized charge equalization apparatus is problematic in that, since a number of secondary windings corresponding to the number of battery cells is wound around a single common core, it is difficult to wind secondary windings around a single common core under optimized conditions.
In detail, in the conventional centralized charge equalization apparatus, a number of secondary windings corresponding to the number of battery cells and wound around a common core must be wound to have the same characteristics. For this operation, a number of secondary windings corresponding to the number of battery cells must be wound to satisfy the same conditions in the relationship with a primary winding, but it is difficult in practice to manufacture a transformer that satisfies these conditions due to the structure thereof.
Further, in the case where a battery pack or battery array, in which battery cells are stacked in multiple layers, is applied to devices that must be portable, devices added to the battery pack or battery array must provide support in order to minimize the size of the battery pack or battery array. However, since the conventional centralized charge equalization apparatus uses a single large-sized common core, it is difficult to integrate the battery pack or battery array.